Fix FTBFS with -Werror=format-overflow
Fix FTBFS with -Werror=format-overflow

i2c.c fails to compile with gcc7 and -Werror=format-overflow used in
Debian Unstable and Ubuntu 18.04 :

i2c.c: In function ‘i2c_init’:
i2c.c:211:15: error: ‘%s’ directive writing up to 255 bytes into a
region of size 236 [-Werror=format-overflow=]

dpath is supposed to store an entire path.

Reported-by: Michel Normand <10b15bce2c7509208f200eeee988f2b52225636e@free.fr>
Signed-off-by: Frédéric Bonnard <ae6f9d84b334adea62ee81ff756fd82a6b0f09ff@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <efc2d7c347a094a6ec1d918179e819039044c564@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

stewart-ibm/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,stewart-ibm/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot